Epilepsy is an important central nervous system disease characterized by recurring convulsive seizures. It has been shown that convulsions arise when an imbalance exists between two principal neurotransmitters, L-glutamic acid, an excitatory neurotransmitter, and γ-aminobutyric acid (GABA), an inhibitory neurotransmitter. Two enzymes, glutamic acid decarboxylase (GAD) and γ-aminobutyric acid aminotransferase (GABA-AT), regulate the level of GABA in the brain. GAD synthesizes GABA from L-glutamic acid. GABA-AT, which is a pyridoxal-5′-phosphate (PLP, 1) dependent enzyme, converts GABA to succinic semialdehyde. During transamination, PLP is converted to pyridoxamine-5′-phosphate (PMP), which is transformed back to PLP by a reaction with 2-ketoglutarate; the 2-ketoglutarate is converted to L-glutamic acid. Succinic semialdehyde dehydrogenase (SSDH) oxidizes succinic semialdehyde to succinic acid using NADP+ as the cofactor (Scheme 1).


When the level of GABA in the brain falls below a threshold level, convulsions occur. Taking GABA orally is not effective in raising the GABA level in the brain because GABA cannot cross the blood-brain barrier. One promising approach is to inhibit GABA-AT, which degrades GABA. The level of GABA would increase due to its continuous production. The catalytic mechanism of GABA-AT is shown in Scheme 2. The cofactor PLP is bound to Lys329 in the form of a Schiff base. (Storici, P.; Capitani, G.; Biase, D. D.; Moser, M.; John, R. A.; Jansonium, J. N.; Schirmer, T. Crystal Structure of GABA-Aminotransferase, a Target for Antiepileptic Drug Therapy. Biochemistry, 1999, 38, 8628-8634.) Transimination gives the imine between GABA and PLP. The enzyme then removes the γ-proton of GABA to give the aldimine, which is subsequently hydrolyzed to produce succinic semialdehyde and PMP.
In the art, it is understood by definition that a mechanism-based irreversible inhibitor is an unreactive compound that has a structural similarity to the substrate or product for the target enzyme and is converted by the target enzyme into a species that inactivates the enzyme prior to its release from the active site. (Silverman, R. B. Mechanism-Based Enzyme Inactivation: Chemistry and Enzymology, Vol. 1; CRC Press: Boca Raton, 1988.) One such inhibitor is the rationally designed vigabatrin (2), an epilepsy drug marketed all over the world, except in the U.S., which irreversibly inhibits GABA-AT by the mechanisms shown in Scheme 3. Pathway a (Michael addition) has been determined and found to account for about 70-75% of the total inactivation. (Nanavati, S. M.; Silverman, R. B. Mechanisms of Inactivation of γ-aminobutyric Acid Aminotransferase by the Antiepilepsy Drug γ-Vinyl GABA (Vigabatrin). J. Am. Chem. Soc. 1991, 113, 9341-9349.)


Previously, 3, a conformationally-rigid vigabatrin analogue, was synthesized.
Surprisingly, 3 was not a GABA-AT inactivator but was a very good substrate (Km=0.1 mM, kcat=11.7 min−1, kcat/Km=117 mM−1 min−1) with a specificity constant almost six times greater than that of GABA (Km=2.4 mM, kcat=49 min−1, kcat/Km=20.4 mM−1 min−1). It was later determined by computer modeling that the endocyclic double bond is not in the right orientation for Michael addition (pathway a, Scheme 3), nor is it an effective enamine for enzyme inactivation. Therefore 7, which has an exocyclic double bond, was designed and prepared from diketone 4, as shown in Scheme 4. (Qiu, J.; Pingsterhaus, J. M.; Silverman, R. B. Inhibition and Substrate Activity of Conformationally Rigid Vigabatrin Analogues with γ-Aminobutyric Acid Aminotransferase. J. Med. Chem. 1999, 42, 4725-4728.) An addition reaction with (trimethylsilyl)methylmagnesium chloride followed by elimination furnished 6. Deprotection of the benzyl group and hydrolysis of the lactam gave the amino acid 7. (Specific reagents and conditions: (a) TMSCH2MgCl, −30° C. to RT, 38%; (b) (CF3CO)2O, DMAP, then TBABr, KF, 86%; (c) Na/NH3/tBuOH,; (d) 2N HCI, 90%, 2 steps.)

Interestingly, 7 inactivated GABA-AT, but when 2-mercaptoethanol was added to the incubation mixture, no inactivation was observed. A possible mechanism accounting for this phenomenon is shown in Scheme 5. It is likely that 7 is only a substrate for GABA-AT. After formation of 8, the double bond is not reactive enough, so this intermediate is not trapped by the enzyme, but rather is released from the active site in the form of an α,β-unsaturated ketone (9). In the presence of 2-mercaptoethanol, a reactive nucleophile, 9 is trapped to form 10, giving no inactivation
of the enzyme. In the absence of 2-mercaptoethanol, however, 9 may return to the enzyme and become covalently attached to the enzyme (11), leading to the enzyme's inactivation. According, however, to definitions, prior art compound 7 is not a mechanism-based inactivator inasmuch as inactivation does not occur prior to the release of the active species from the active site.